Hello, I understand your worry about pregnancy probability. If the first day of your menses was on March 1st and you had unprotected sex 7 days after and took plan B as well, in my opinion there would be little chance for you to become pregnant.
Firstly because you were not in your ovulation window yet and secondly since you took Plan B this would make it even less probable.
If you would like to protect yourself from unwanted future pregnancies, I would suggest you to consult your health care provider for more information about contraceptives and he or she would kindly explain them to you and suggest you a good choice. I wish you the best.
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